Alveolar rhabdomyosarcoma (aRMS) is a highly malignant tumor and it harbors a unique t (2;13) chromosomal translocation marker that leads to the formation of PAX3-FKHR fusion transcription factor, As the result of fusion process, PAX3-FKHR gains the ability to regulate expression of genes that are not normally regulated by Pax3 and FKHR. This altered gene targeting property is responsible for the oncogenic action of PAX3-FKHR. Therefore, understanding the basis of tumor-specific activation/inactivation pathways will provide useful determinants in therapeutic designs for patient care management.The objective of the current grant proposal is to characterize tumor-specific regulatory mechanisms that contribute to the malignant phenotype in PAX3-FKHR transformed cells. Two lines of evidence from the preliminary study suggest that protein proteolysis plays an important role in PAX3-FKHR mediated oncogenesis. One, PAX3-FKHR aberrantly activates stromelysin-1 1 gene expression. Abnormal expression of stromelysin-1 1 gene is closely associated with tumor invasion and metastatic phenotypes, a characteristic that is found predominantly in the aRMS. Second, PAX3-FKHR accelerates 26Sproteosome dependent degradation of cyclin kinase inhibitor p27Kipl protein, a key regulator involved in cell cycle regulation. Aim 1 of this research is designed to gain a better understanding of regulatory mechanism involved in stromelysin-1 gene activation by PAX3-FKHR and the functional role of stromelysin-1 1 in PAX3-FKHR dependent tumorigenesis. This will provide a mechanistic link between the fusion protein and invasive and metastatic clinical behavior in aRMS tumors. Aim 2 focuses on characterizing the molecular steps involved in PAX3-FKHR down-regulation of p27kipl protein accumulation. Finally, in Aim 3, we will examine the role of deregulation of cell cycle regulatory proteins by PAX3-FKHR in its ability to inhibit myogenic differentiation. Studies outlined in aims 2 and 3 will provide a molecular basis for the uncontrolled growth and differentiation phenotypes in aRMS cells.The proposed studies will further our understanding of mechanistic association between genetic abnormality and pathogenesis of aRMS; they will also expand our ability to use this information for patient diagnosis/or prognosis and to design therapeutic reagents that specifically interrupt tumor function without damaging normal cell function.